The invention relates on the one hand to a male screw thread for a threaded member to be screwed into a female screw thread in a cooperating member more particularly in the form of a cylindrical or taper Withworth pipe thread, a BSP thread, Japanese pipe thread (PT) or National pipe thread (NPT) such as for the production of connections in pneumatic systems. On the other hand the invention relates to a female screw thread serving the same purpose and suitable for receiving a male screw thread on a male member which more particularly has a cylindrical or taper section in the form of a Withworth pipe thread, a BSP thread, a Japanese pipe thread (PT) or a National pipe thread (NPT), the female screw thread having a taper section at the front axial end which is to be screwed in and tapering in the direction of insertion from a cylindrical screw section of the male thread.
In the field of pneumatic and hydraulic systems it is necessary to use many different types of screw connections for the production of systems containing the working fluid. This may for instance be to produce connections with fluid power pipes, fluid power hose, tubes or the like components with pneumatic or, respectively, hydraulic means such as piston and cylinder units or valves or only to shut off a port in such a device by means of a plug.
So far there has been the disadvantage that different countries prescribe different types of screw threads or different standards so that the individual component and pieces of equipment have to be fitted with types of screw thread dependent on the location of use. The most frequent types of screw thread are the National pipe thread (NPT, USA), the Japanese pipe thread (PT), and the British standard pipe thread (BSP) with a taper and cylindrical form and which practically corresponds to the taper (DIN 2999) or cylindrical (ISO 228/1) Withworth pipe thread, which is the pipe thread overwhelmingly employed in Germany. It is clear that the production of components with such a wide variety of types of screw thread is very expensive. Furthermore confusion is likely as for instance by mistakes of the supplier which are only noticed later during use or if not noticed may entail stripping of the screw threads when an attempt is made to screw parts together. Such screw joints may lead to leaks.
In order to overcome these difficulties there has already been a proposal in the U.S. Pat. No. 4,033,615 to provide a female screw thread of the type initially mentioned which is to make possible connections with parts having NPT or BSP thread. This system does however leave to be desired, because the hybrid pitch angle provided along the full length and the averaged flank angle lead to substantial problems, more particularly when screwing into cylindrical Withworth or BSP threads. Such screw threads do not normally seal in the thread itself and in fact make necessary the use of additional gasket rings, which have to be swaged against an abutment surface. If however the threads jam when they have already been screwed together, it is not possible to ensure that there is optimum pressing of the gasket ring with the required degree of certainty.